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All birds reproduce by means of eggs, unlike other vertebrates such as some reptiles and most mammals, which are viviparous. Bird eggs have a calcareous shell, which protects both the embryo and the substances that nourish it from external environmental factors. This has allowed them to colonize environments as diverse as desert, forests, wetlands, plains, mountains and even places with permanent ice like Antarctica.
The eggs vary from one species to another in their shape, texture, color, and size. The variations are exemplified here with the life-size silhouette of the African Ostrich and Hummingbird eggs.
Inset: Nest of Zorzal Chiguanco (Turdus chiguanco) with eggs and adult incubating in the same nest.